What is a Trojan Horse?

A TROJAN HORSE is a program that does something undocumented that the
programmer intended, but that some users would not approve of if they
knew about it. According to some people, a virus is a particular case
of a Trojan Horse, namely one which is able to spread to other programs
(i.e., it turns them into Trojans too). According to others, a virus
that does not do any deliberate damage (other than merely replicating)
is not a Trojan. Finally, despite the definitions, many people use the
term "Trojan" to refer only to *non-replicating* malware, so that the
set of Trojans and the set of viruses are disjoint.